The present invention relates to a process for the coherent detection and demodulation of a phasemodulated carrier wave in a random polarization state and to an apparatus for performing this process. It is used in the optical transmission of data, particularly by optical fibres.
An optical transmission system generally comprises a transmitter, in which an optical wave is modulated by a signal containing the data to be transmitted, a transmission line (generally a single-mode optical fibre) and a receiver in which the transmitted wave is demodulated. The detection operation can be of the coherent type in the sense that it uses a coherent optical wave which is struck with the modulated carrier wave and produced by an appropriately controlled local oscillator.
This so-called coherent detection procedure is described in numerous articles and reference is made, for example, to the article by F. FAVRE et al entitled "Progress Towards Heterodyne-Type Single-Mode Fibre Communication Systems", published in the Journal of Quantum Electronics, Vol. QE-17, No. 6, June 1981, pp. 897 to 905.
Coherent detection requires a stable polarization state of the wave to be demodulated. However, this is not generally the case, due to disturbances suffered by the wave during its propagation in the fibre. It is then necessary to provide means for correcting this instability. A first solution which has been proposed consists of using a linear or circular polarization maintaining fibre, as described in the article by L. JEUNHOMME et al entitled "Polarisation-maintaining single-mode fibre cable design" published in Electronics Letters, Nov. 20th 1980, Vol. 16, No. 24, pp. 921/2. Another known solution consists of the stabilization of the polarization by feedback, as described in the article by ROD. C. ALFERNESS entitled "Electrooptic Guided-Wave Device for General Polarization Transformations", published in the IEEE Journal of Quantum Electronics, Vol. QE-17, No. 6, June 1981, pp. 965/9 and in the article by R. ULRICH entitled "Polarization stabilization on single-mode fiber" published in Applied Physics LEtters, 35 (11), Dec. 1st 1979, pp. 840/2.
However, both these solution types suffer from disadvantages, such as additional losses due to the special structure of the polarization maintaining fibre and possibly ageing problems, or large insertion losses in the case of active polarization control. In both these solutions, the increase of the losses is at least 10 dB, even in the optimum case.
Moreover, the polarization maintaining fibre solution causes problems with regards to connections between fibres and as described in the article by M. MONNERIE entitled "Polarization-maintaining single-mode fiber cables: influence of joins" published in Applied Optics, Vol. 20. No. 14, July 15th 1981.